1. Field of the Invention
The present invention relates to a lead frame material and a process for manufacturing the same, and more particularly to a process for forming a lead frame coating by vacuum deposition. By using vacuum deposition, the coating material can be selected from a wider variety of materials. Coatings with novel compositions can thus be formed.
2. Description of the Prior Art
Copper-based lead frame substrates are dominantly employed in semiconductor device packages primarily because of their superior conductivity. In order to improve copper-based lead frame substrates, one common approach is to enhance the wire bonding, reliability, or solderability of the metal conducting coating on the lead frame substrate, which is called lead frame coating.
Optimal lead frame coatings should be free of lead (Pb) and lead compounds, corrosion resistant, oxidation resistant, resistant to high temperatures (over 200.degree. C.), wire-bondable, and solderable. Many attempts have been made to meet the desirable properties described above. One method is to carry out the wire bonding in vacuum. Another method is to form a gold layer of 5.about.10 .mu.m thickness or to remove oxides on the surface of the lead frame coating. However, for those conventional methods, production steps and cost are both increased. Moreover, for a high pin lead frame, the neighboring leads will stick together when the lead frame coating is too thick.
Nowadays, the deposition of a lead frame coating such as Cu/Ag, Ni/Ag, and Pd on a lead frame substrate is employed by wet electroplating. The main raw materials for electroplating are cyanides. Since cyanides have high toxicity, they are harmful to humans in lead frame factories, and the waste liquid causes serious environmental problems.
In Taiwan, the deposition of the lead frame coating on a lead frame substrate is also primarily conducted by wet electroplating. Lead frame factories consume a large amount of deionized water or tap water to wash away the residual waste liquid on the lead frame generated by electroplating. For one piece of lead frame, about 0.5 kg to 1 kg of deionized water or tap water is consumed for washing. For a medium-sized lead frame factory, which can produce about 200 million pieces of lead frames per month, the amount of water required for washing is about 200 thousand tons per month.
In addition, it is very difficult to make the current density of the plating solution uniform. Thus, lead frames suffer from many defects including non-uniform color, burnt, incomplete plating, pits, nodules, voids, pin holes, deformation, wrinkles, rust, stains, oxidation, water stains, chemical pollution, discoloration, scratches, silver leakage, silver stains, and silver immersion.
The deposition of lead frame coatings by wet electroplating has other disadvantages, as described below. The silver and copper formed by wet electroplating is very easily oxidized. When the plating solution is polluted, the plating surface will also be polluted and discolored, thus causing difficulty for wire bonding. In the 10 to 20 meter long electroplating production line, the products are very easy to scratch. The products will have water stains after washing with water. Moreover, when the production line is shut down temporarily because of unusual conditions, all the finished and semi-finished products will be immersed in the plating solution or water for a period of time. Thus, the products will be oxidized and discolored, making them useless. In addition, all the plating solution on the production line will need to be changed, making the total cost very high.
In recent years, many researchers have attempted to improve the properties of the lead frame coating by means of changing the plating composition. For example, Takahashi in U.S. Pat. No. 5,510,197 provides a new lead frame material including a base plate of copper or copper alloys, an intermediate layer composed of nickel or nickel alloys, and a protective coating composed of gold, silver, palladium and their alloys. Moon in U.S. Pat. No. 5,750,016 provides a process for plating palladium or its alloy onto an iron-nickel alloy substrate. Mathew in U.S. Pat. No. 5,650,661 provides a new lead frame material including a lead frame substrate, a nickel layer, a copper layer, a tin layer, and a palladium layer in sequence, which are all formed by electroplating. However, the varieties of plating solution for electroplating are limited in number; therefore, the number of suitable plating compositions is limited accordingly.
In summary, deposition of lead frame coating by wet electroplating has the problems of environmental pollution, high cost, low yield, and a limited number of suitable coating compositions.